Dark Matter and New Physics
The Fermi-Large Angle Telescope (Fermi-LAT) probes photons of the highest energies. At such energy scales, these particles may exhibit signatures of the new physics, which deviate significantly from the Standard Model. For instance, the currently accepted standard cosmological model predicts that the universe is composed of roughly 24 percent non-baryonic (i.e., not composed of quarks) dark matter (DM). So far the existence of dark matter has been inferred only from its gravitational influence on large scales and several experimental techniques for detecting the dark matter particle are currently being pursued including direct searches with terrestrial detectors, production at an accelerator such as the Large Hadron Collider (LHC), and indirect detection through the measurement of the secondary products of dark matter annihilations or decays.
Many candidates have been proposed for the dark matter particle, such as the neutralino of Supersymmetry (SUSY), the Kaluza-Klein particle in Universal Extra Dimensions (UED), and the axion motivated by the strong-CP problem in Quantum Chromodynamics (QCD). These candidates are compelling because each was originally proposed to address theoretical problems in domains of physics completely outside the realm of cosmology.
For instance, the strong-CP problem refers to the observed absence of charge-parity (CP) violation in the strong force interactions, which is mediated by gluons, in contrast to the CP violation observed in electroweak force interactions. In some of these models, the dark matter particle may self-annihilate or decay into standard model particles, including photons with energies as large as the dark matter particle rest mass. The detection of secondary gamma-rays from these processes with the Fermi-Large Angle Telescope (LAT) could provide compelling evidence for the dark matter particle.
One of the most promising sites in the search for dark matter is the center of our own galaxy. Based on current models, this region is expected to have a high-density dark matter cusp in its center. However the galactic center is also one of the most complex and difficult regions of the sky to model because of the strong diffuse emission and high density of gamma-ray sources. Fermi-LAT team members at KIPAC are working to better understand the gamma-ray data from the galactic center region. This effort might yield the discovery of a dark matter signal or, in the absence of a signal, to key clues on the nature of dark matter.
A preliminary analysis of the large-scale gamma-ray emission from the Milky Way has revealed the existence of large (possibly galactic scale) structures in the direction of the inner part of our galaxy (see Su, Slatyer, and Finkbeiner 2010 ApJ, 724:1044-1082). The origin of these structures is not yet known and under investigation. Although so far there is no clear indication that this excess originates from dark matter annihilation or decay, its contribution needs to be understood before attempting to disentangle a dark matter signal from the inner regions of the Milky Way .
Other signatures of dark matter annihilation that could be detected by the Fermi-LAT include the annihilation from so-called dark matter satellites such as the dwarf spheroidal galaxies or completely dark substructures, gamma-ray lines, as well as the diffuse gamma-ray emission from the Milky Way halo and extragalactic dark matter.
Signatures of dark matter annihilation in the Milky Way halo and extragalactic dark matter may be hidden in the extragalactic gamma-ray background (EGB). The EGB is an isotropic component of the gamma-ray sky, which is thought to be composed of many unresolved point sources. However, analysis of the EGB has found a significant excess of gamma-ray emission even after the subtraction of the expected contribution from known point-source populations, such as active galactic nuclei (AGN) and star-forming galaxies. This excess could be a signature of dark matter annihilation in galactic dark matter substructures or dark matter halos beyond our own galaxy.
For recent Fermi papers on these topics see: Abdo et al. 2010 JCAP, 4, 14, Abdo et al. 2010 ApJ, 712:147-158, Abdo et al. 2010 Phys. Rev. Lett., 104, 091302
Residual map of the Fermi-LAT gamma-ray sky in galactic coordinates after subtraction of known point sources and diffuse emission. Two lobe-like structures aligned with the Galactic Center and extending to more than 50 degrees in galactic latitude are apparent in these residuals. The relationship of these structures as well as the gamma-ray emission from the galactic center to the expected signatures of dark matter annihilations is currently an active area of investigation.
Illustration of the different strategies used to search the signature of dark matter in the Fermi-LAT data. The image shows the gamma-ray sky from dark matter annihilations in the galaxy as predicted by the Via Lactea II N-body simulation (Pieri et al. 2011 Phys.Rev.D83:023518). The central peak in the luminosity distribution is due to the emission from the smooth component of the Galactic halo. The smaller regions of emission are individual dark matter substructures. The Fermi-LAT can potentially detect both the smooth and clumped component of the dark matter annihilation signal. Part of the challenge of this work is the uncertainty in the energy distribution of the annihilation gamma-rays owing to the unknown mass of the dark matter particle and the range of possible annihilation channels.
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